Despite recent advances, there remains a tremendous need for better detection methods for measuring gas phase molecules and substances. Measurement techniques exhibiting greater reliability, reproducibility, and sensitivity are desired, particularly if they can be achieved using cost-effective sensors. For example, some analytical situations require high sensitivity devices to monitor low concentrations of volatile substances that may indicate the presence of toxic, explosive, corrosive, combustible, or otherwise dangerous materials. Other situations demand methods of high selectivity to determine the presence of a single molecular species in medical, environmental, engineering applications, without interference from other molecules. More often than not, both enhanced sensitivity and selectivity are preferred to provide the most useful and timely analytical information.
Numerous technical applications currently using standard analytical methods for trace organic and inorganic gases would benefit significantly from enhanced detection means. For example, environmental protection applications such as emissions testing, EPA compliance studies, or chemical analyses of effluent streams, require more selective and sensitive measurement techniques. More rugged and reliable field sensors that provide improved sensitivity, yet are sufficiently inexpensive and portable for routine use, would be particularly useful.
Diagnostic medical applications also require better detection methods, where certain volatile compounds indicative of a particular medical condition must be measured. For example, compounds or their byproducts indicative of a medical condition can be exuded in low concentration through the skin, from wounds, in perspiration, or occur in the breath, and therefore require reliable and highly sensitive analytical techniques for their measurement. An improved sensor is also needed for monitoring the concentration of anesthetics, or their metabolic breakdown products, as they emanate from the skin of a patient under anesthesia.
More convenient, rapid and accurate detection methods are also needed to test for the presence of alcohol, drugs, or drug byproducts in the breath of a motorist or an athlete. Such methods would be especially useful to test truck drivers, bus drivers, train engineers, ship and barge captains, and heavy equipment operators, where liability issues arise.
Improved analytical techniques are urgently needed in security applications such as airport screening and the protection of government buildings, where explosive substances can be detected by the presence of diagnostic volatile compounds. These applications are in dire need of reproducible, sensitive, and cost-effective methods for molecular detection. Home and work place security applications, where gas detection is related to both comfort and safety, have similar requirements. Similarly, suspicious areas in which land mines may occur might be identified, and mines located, through detecting diagnostic volatile compounds.
Rapid and reliable security methods are urgently needed at ports of entry to monitor the massive volume of container traffic that enters these ports in ships or across borders on trucks. It is highly desirable that every container entering the country be subject to analytical tests capable of detecting explosives, dangerous materials, or precursors to harmful substances. What is therefore needed is a test for the presence of diagnostic volatile compounds indicative of these materials, rapid enough to afford the high throughput required to test every single container.
Continuous ambient air monitoring in electronics manufacturing and storage facilities also requires enhanced analytical methods, where maintaining the integrity of the atmosphere requires a rapid and selective means of detecting contaminants in the air. Air quality control is especially important in the electronics industry to prevent damage to sensitive electronic components stored within the confines of a manufacturing facility, where the ambient air may contain harmful levels of vapors produced or used in that facility. One aspect of the electronics industry where monitoring corrosion is critical is the manufacture of magnetic recording data storage systems such as disk drives.
Air quality monitoring in archival repositories also requires improved detection methods and devices. Accurate air quality measurements must be implemented along with rigorous air purification to insure proper storage conditions for sensitive materials such as archival documents, films, photographs, lithographs, historic books and manuscripts, maps, and the like.
Further, there is a great need to protect personnel in government buildings, embassies, defense command and control areas, and even temporary field operations, against chemical or biological warfare agents, particularly during war or terrorist attacks. A technique that could be adapted to determine the presence of either chemical or biological agents, or both simultaneously, would be especially useful.
Currently, detection and measurement of volatile substances is performed by any number of methods, all of which suffer from various limitations in sensitivity, selectivity, ease of operation, or cost-effectiveness. For example, combustion-type molecular detectors currently in use employ a catalyst coating bound directly to a resistive wire, for example, alumina-supported platinum metals such as Pt, Pd or Rh on a platinum wire, which is heated up to several hundred degrees Celsius. When the heated catalyst contacts the target gas, the heat of combustion increases the temperature of the platinum wire, which is detected as a voltage change, resulting from a change of the electrical resistance of the wire in response to the temperature increase. However, correct measurements are difficult, due in part to the difficulty in accurately quantifying a comparatively small temperature increase (ΔT) at a high temperature (T). Further, the resistive wire is prone to electromagnetic interference and is subject to physical movement and turbulence within the air stream, resulting in signal noise. Chemical poisoning of the supported metals may also result in unreliable results.
A related type of sensor for gas phase molecules in common use is the resistance-type sensors utilizing a metal oxide, especially an n-type semiconductor oxide such as SnO2, and often supported on ceramic beads. These detectors operate on the basis of catalytic oxidation of a target molecule by adsorbed oxygen, with a concomitant reduction of the semiconductor oxide, and are often used for measuring the combustible hydrocarbons or CO in automobile exhaust. The change in resistance of the sensing element resulting from oxygen desorption, upon oxidation of the combustible gas, is used as a proxy for gas concentration. However, presently available sensors are susceptible to numerous interfering compounds such as such as alcohols, humidity, Si-containing compounds, other volatile organic compounds, and even varying oxygen levels, resulting in inaccurate and non-reproducible results. Chemical poisoning of the SnO2 may also be problematic. Further, the resistance of the semiconductor itself varies at high temperatures, further rendering the results unreliable.
Some gas sensors are designed to detect a specific type of gaseous molecule only, and therefore are not generally applicable. For example, one type of detector relies on a proton-conductive layer which functions to dissociate and thereby detect, hydrogen or other proton-releasing molecules. However, such a detector is adapted only for measuring proton-releasing molecules. Similarly, some air-fuel ratio sensors that detect O2 use an oxygen ion conductive solid electrolyte detector. This device is adapted only for measuring molecules that form oxygen ions upon contact with the electrolyte. Moreover, such detectors typically require very high operating temperatures (up to about 700° C.).
Some gas detectors are based on very explicit chemical reactions or specific spectroscopic properties of the target molecule, as in the case of some conventional NOx analyzers. For example, detection may be accomplished by chemical luminescence or by gas-phase infrared or Raman spectral analysis of various vibrational chromophores of a target molecule. Such methods are typically not readily adapted for directly situating the detecting element into a fluid stream, and therefore are not suitable for analyzing transient gas concentrations, a needed capability when combining detection with electronic controls, such as in automobile emissions systems under feedback control. These systems may also require frequent maintenance of optical components, further reducing their utility.
Other devices used for the identification of molecular contaminants rely on simple changes in the thermal conductivity of the gas being examined. However, thermal conductivity is a macroscale measurement that evaluates any mixture of gases with which the detector is presented. Such devices are not capable of discriminating among discrete molecules, but rather provide qualitative rather than quantitative measurements. As a result, their utility is severely limited and would not, for example, be able to distinguish the thermal conductivity component of a single gas such as a single metabolic gas or a single component in cigarette smoke.
Fuel cell technologies have also been utilized in the detection of specific molecules, particularly when the target appears in low concentrations. However, this technique is often ineffective because the chemical reaction driving the fuel cell reaction can be nondiscriminatory, compromising the ability of this method to distinguish among multiple molecular species.
It has therefore become imperative to address the present limitations associated with gas phase molecular detection by providing new devices and new methods for detecting, identifying, and quantifying gaseous substances. The new systems would preferably utilize a fundamentally new method of detection that affords enhanced selectivity, while retaining the necessary sensitivity. The present invention addresses these problems by providing novel sensors and methods for selectively identifying and measuring gaseous substances. The new sensors achieve high sensitivities, allowing the detection of gas phase species at very low concentrations, and greatly expanding their applicability. The new sensors are also highly selective, able to distinguish a single molecular species while ignoring all others. This capability which makes this invention especially useful in critical analytical areas such as security and medical applications. This improved selectivity results in highly reliable measurements and significantly reduces the cross-sensitivity from interfering species. This invention also provides new analytical paradigms for detecting and measuring multiple target substances simultaneously and with high reproducibility. Further, the sensors and methods of this invention are relatively simple as compared to many of the current technologies, thereby providing a more error-free operation and significantly greater cost-effectiveness in return.